The present invention relates to code-division-multiple-access (CDMA) communications, and more particularly to a method for fast modulation using the Fast Hadamard Transform in a synchronous CDMA antenna array wireless system.
In PCS and other wireless communication systems, a central base station communicates with a plurality of remote terminals. Frequency-division multiple access (FDMA) and Time-division multiple access (TDMA) are the traditional multiple access schemes to provide simultaneous services to a number of terminals. The basic idea behind FDMA and TDMA techniques is to slice the available resource into multiple frequency or time slots, respectively, so that multiple terminals can be accommodated without causing interference.
Contrasting these schemes which separate signals in frequency or time domains, Code-division multiple access (CDMA) allows multiple users to share a common frequency and time channel by using coded modulation. In addition to bandwidth efficiency and interference immunity, CDMA has shown real promise in wireless applications for its adaptability to dynamic traffic patterns in a mobile environment. Because of these intrinsic advantages, CDMA is seen as the generic next-generation signal access strategy for wireless communications.
In CDMA communications, the message information is modulated by a pseudo noise (PN) sequence or spreading code. Synchronous CDMA (S-CDMA) communications systems provide for synchronous transmission and reception by all users. S-CDMA thus allows the use of orthogonal code words which possess inherent mutual interference resistance. In contrast, asynchronous CDMA systems do not provide for synchronous transmission and thus cannot use orthogonal code words. Thus S-CDMA systems offer high capacity relative to asynchronous CDMA systems with the same spreading gain. An ideal application of S-CDMA scheme is one-point to multi-point (point-to-multipoint) communications, e.g., from a base station to a plurality of remote terminals, where signals transmitted to different terminals are perfectly synchronized.
When an antenna array is employed at the base station, spatial diversity of a wireless system can be exploited to enhance system performance. Among the advantages inherent to an antenna array S-CDMA system are large capacity and range, strong resistance against multipath fading and interference, superior traffic adaptability, and highly flexible hand-off capabilities. There have been a significant number of patents in the area of antenna array applications for wireless communications including V. Graziano, "Antenna Array for a Cellular RF Communications System," U.S. Pat. No. 4,128,740, 13/1977.
In addition to the spreading and combining processes well-known in the current art of CDMA, modulation in an antenna array CDMA system also involves transmission beamforming. Transmission beamforming involves determining the transfer function of the transmission path of a respective terminal, referred to as a spatial signature estimate. The spatial signature estimate is then used to determine transmission beamforming coefficients that are used for transmission to the terminal. The transmission beamforming coefficients represent different weighting values corresponding to each of the antennas in the antenna array. Each transmission beamforming coefficient is multiplied with a user's modulated PN sequence, and the respective results are provided to the respective antennas in the array.
In summary, modulation in an antenna array CDMA system can achieved by (i) spreading individual message symbols into chip sequences with pre-assigned orthogonal code words (the Walsh Codes); (ii) weighting (beamforming) each chip sequence with a transmission beamforming vector to produce a multichannel chip sequence; and (iii) mixing all multichannel chip sequences to generate transmission multichannel chip sequences for pulse shaping and RF upconversion. The order of (i) and (ii) may be switched without affecting the final results.
Demodulation in a smart antenna CDMA system can be realized using the computationally efficient Fast Hadamard Transform (FHT). In contrast to demodulation, modulation in S-CDMA, especially antenna array S-CDMA, is a more involved process. The spreading and mixing, all at the chip level, are particularly computationally demanding. Please refer to E. Zehavi, "Method and Apparatus for Bifurcating Signal Transmission over In-phase and Quadrature Phase Spread Spectrum Communication Channels", U.S. Pat. No. 5,414,728, 5/1995, for more details on chip-level S-CDMA modulation. The modulation process is further complicated because of beamforming, which allows each beamformed chip signal to take an arbitrary value instead of from a finite alphabet.
A system for cell-site CDMA modulation is disclosed in U.S. Pat. No. 5,300,474, entitled "System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System". The techniques disclosed therein utilize multiple modulators and D/A converters, each corresponding to one user, and an analog combiner to realize baseband modulation. While this implementation alleviates the computational load of the digital portion of the system, this method introduces undue hardware and clearly increases the system cost and complexity. In a standard software radio design where even dedicated tasks are exported to the digital processor section, it is desirable to perform the modulation using general DSPs rather than customized baseband processors. However, the large volume of data involved due to multiuser, multichannel spreading imposes an extreme challenge to DSP implementation.
An effective way to reduce the required computations in modulation is by using look-up tables (LUTs), i.e., constructing a table whose elements are the ready-to-transmit signals corresponding to all possible symbol combinations. The idea is to centralize the computational load using dedicated DSPs within a certain period so that during actual transmission, signal modulation is reduced to fetching pre-calculated values from the LUT storage. This straightforward approach to reducing complexity requires, however, an extremely large amount of storage in present applications, and a correspondingly large computation time in the construction of the look-up tables.
In addition, as mentioned above, in a smart antenna system the modulation process is further complicated because of beamforming, which allows each beamformed chip signal to take an arbitrary value instead of from a finite alphabet. In other words, the beamforming vector values can take on any value, and thus the beamformed or weighted chip sequences can also take on any value. Since the resultant beamformed or weighted chip sequences can take on any value, look-up tables cannot be used for modulation in a smart antenna system.
Therefore, prior art systems which perform the aforementioned modulation tasks generally introduce undesirable hardware or computational complexity. It is therefore an object of the present invention to provide a system and method for fast modulation of CDMA signals in an antenna array system.